Carbonate-promoted carboxylation reactions for the synthesis of valuable organic compounds

ABSTRACT

A method for synthesizing a dicarboxylate containing an aromatic heterocycle is provided. A carboxylic acid containing a heterocycle is provided. A CO32− salt is provided to form a mixture, which converts the carboxylic acid containing an aromatic heterocycle to a carboxylate containing an aromatic heterocycle. CO2 gas is provided to the mixture. The mixture is heated to a temperature to at least partially melt the carboxylate containing an aromatic heterocycle.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 15/558,528 filed Sep. 14, 2017, which is a 371 of International Application No. PCT/US16/022969 filed Mar. 17, 2016. This application claims priority under 35 U.S.C. § 119 from U.S. Provisional Patent Application No. 62/136,288, filed Mar. 20, 2015, entitled Carbonate-Promoted Carboxylation Reactions for the Synthesis of Valuable Organic Compounds, which is incorporated herein by reference for all purposes.

BACKGROUND

This disclosure relates generally to the reduction of CO₂. More specifically, this disclosure relates to the formation of carbon based chemicals and carbon based fuels using CO₂.

SUMMARY

In accordance with this disclosure, a method for synthesizing a dicarboxylate containing an aromatic heterocycle is provided. A carboxylic acid containing a heterocycle is provided. A CO₃ ²⁻ salt is provided to form a mixture, which converts the carboxylic acid containing an aromatic heterocycle to a carboxylate containing an aromatic heterocycle. CO₂ gas is provided to the mixture. The mixture is heated to a temperature to at least partially melt the carboxylate containing an aromatic heterocycle.

The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a schematic illustration of a CO₃ ²⁻-promoted C—H carboxylation reaction.

FIG. 1b is a schematic illustration of synthesis of FDCA from inedible biomass.

FIG. 2a is a schematic illustration of the conversion of furan-2-carboxylate into furan-2,5-dicarboxylate.

FIG. 2b is a schematic illustration of the conversion of furan-2-carboxylate into furan-2,5-dicarboxylate using potassium (K⁺) salts.

FIG. 2c schematically illustrates H/D isotopic exchange between furan-2-carboxylate and cesium acetate-d3.

FIG. 3a schematically illustrates the carboxylation of cesium benzoate.

FIG. 3b schematically illustrates CO₃ ²⁻-catalyzed H/D isotope exchange between differentially labeled benzoates.

FIG. 3c schematically illustrates carboxylation of benzene in the presence or absence of Cs⁺ isobutyrate.

FIG. 4 schematically illustrates the isolation of FDCA by protonation with strong acid.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

A major impediment to synthesizing compounds from CO₂ is the difficulty of forming carbon-carbon (C—C) bonds efficiently. CO₂ reacts readily with carbon-centered nucleophiles, but generating these intermediates has previously required high-energy reagents (e.g. highly reducing metals or strong organic bases), carbon-heteroatom bonds, and/or relatively acidic C—H bonds. These requirements negate the environmental benefit of using CO₂ as a substrate and limit the chemistry to low-volume targets. This specification shows that intermediate temperature (200° C.−350° C.) molten salts containing alkali cations (e.g. Cs⁺ or K⁺) enable carbonate (CO₃ ²⁻) to deprotonate very weakly acidic C—H bonds (pKa >40), generating carbon-centered nucleophiles that react with CO₂ to form carboxylates (C—CO₂ ⁻).

To form C—C bonds to CO₂ without using high-energy reagents or specialized substrates, we envisioned a CO₃ ²⁻-promoted C—H carboxylation reaction, wherein CO₃ ²⁻ reversibly deprotonates a C—H bond to generate HCO₃ ⁻ and a carbon-centered nucleophile that reacts with CO₂ to form C—CO₂ ⁻, as shown in FIG. 1a . HCO₃ ⁻ decomposition results in a net consumption of one-half equivalents of CO₃ ²⁻ and CO₂ per C—CO₂ ⁻ produced. The cycle could be closed by protonating C—CO₂ ⁻ with strong acid and using electrodialysis to regenerate the acid and base, effecting a net transformation of C—H and CO₂ into C—CO₂H without using any other stoichiometric reagents. Alternatively, CO₂-promoted esterification could be used to convert the carboxylate into an ester (C—CO₂R) and regenerate CO₃ ²⁻ directly. Previously, researchers have used Cs₂CO₃ to promote C—H carboxylation in organic solvents. However, this approach can only be used to carboxylate relatively acidic C—H bonds such as alkynyl C—H bonds (Dingyi et al. Green Chem. 13, 1275-1279 (2011)), the alpha C—H bond in butenoate (Kudo et al. J. Jpn. Petrol. Inst. 38, 48-51 (1995)), and heteroaryl C—H bonds with pKa up to 27 (Vechorkin et al. Org. Lett. 12, 3567-3569 (2010)). This restriction severely limits the scope of molecules that can be carboxylated. Utilizing CO₃ ²⁻-promoted C—H carboxylation for commodity synthesis requires deprotonating C—H bonds that are considerably less acidic.

A method reported by Raecke (U.S. Pat. No. 3,359,310) described the synthesis of potassium malonate by heating potassium acetate and potassium carbonate under high CO₂ pressure (10-2000 atm) at 200-450° C., but this method is limited to the synthesis of malonate. Raecke does not describe a general method for carboxylating C—H bonds in molecules unrelated to acetate. Another report by Kudo et al (J. Jpn. Petrol. Inst. 38, 40-47 (1995)) described the synthesis of aromatic dicarboxylic acid salts and tricarboxylic acid salts by combining cesium salts of aromatic carboxylic acids, cesium formate, and cesium carbonate under 400 atm of CO₂ and heating to 380° C. The use of very high CO₂ pressures makes the chemistry impractical. Kudo's conditions are intended to promote a reaction between CO₂ and an aromatic ring directly without first deprotonating a C—H bond on the substrate. This approach necessitates the very high CO₂ pressures. The following examples illustrate the generality and utility of CO₃ ²⁻-promoted C—H carboxylation as depicted in FIG. 1 a.

In one embodiment, CO₃ ²⁻-promoted C—H carboxylation followed by protonation is used to convert 2-furoic acid into furan-2,5-dicarboxylic acid (FDCA). FDCA is a highly desirable bio-based feedstock with numerous applications including the synthesis of polyethylene furandicarboxylate (PEF), which is a potential large-scale substitute for petroleum-derived polyethylene terephthalate (PET). In particular, PEF has been reported to have superior physical properties to PET (Burgess, et al. Macromolecules, 47, 1383-1391 (2014)), a commodity polymer produced on a scale of many megatons per year (Mton yr⁻¹). A longstanding goal of renewable plastic synthesis is a scalable synthesis of FDCA from inedible biomass (lignocellulose), as shown in FIG. 1b . Current approaches to FDCA synthesis use dehydration processes to convert hexose sugars into hydroxymethyl furfural (HMF), which is then oxidized to form FDCA. Recent work has significantly improved the efficiency of converting fructose to HMF. However, producing FDCA on a scale commensurate with terephthalic acid and achieving maximal reduction in CO₂ emissions will require using lignocellulose as the feedstock. Converting lignocellulose into HMF is very challenging because the hexoses are incorporated into intractable cellulose fibers. An economical, large-scale lignocellulose-to-HMF process has not been demonstrated. (Sheldon Green Chem. 16, 950-963 (2014)).

In contrast to fructose, the conversion of lignocellulose to furfural has been performed industrially on a ˜400 kton yr⁻¹ scale for decades (Lange et al. ChemSusChem 5, 150-166 (2012)). Furthermore, several methods are available for oxidizing furfural to 2-furoic acid (Hoydonckx et al. Ullmann's Encyclopedia of Industrial Chemistry, 16, 285-313 (2007)); (Van Haveren et al. WO 2013/096998 A1)). At present, however, the available methods for converting 2-furoic acid into FDCA are inefficient, unselective, and consume stoichiometric amounts of energy-intensive reagents (Thiyagarajan et al. RSC Adv., 3, 15678-15686 (2013)). CO₃ ²⁻-promoted C—H carboxylation could be used to convert 2-furoic acid into FDCA and thereby open a new route to PEF utilizing a lignocellulose-derived monomer that is already produced industrially.

The CO₃ ²⁻-promoted C—H carboxylation reaction required for FDCA synthesis is the conversion of furan-2-carboxylate into furan-2,5-dicarboxylate (FDCA²⁻), as shown in FIG. 2a . Assuming it is similar to an un-substituted furan, the pKa of the C—H at the 5-position of furan-2-carboxylate is ˜35. Deprotonation of this C—H has previously required lithium diisopropylamide or n-butyllithium. We hypothesized that CO₃ ²⁻ would deprotonate furan-2-carboxylate if the reaction were performed in a molten salt with a high concentration of alkali cations to stabilize the conjugate base by ion pairing. To test this hypothesis, we attempted C—H carboxylation with mixtures consisting of an alkali metal salt of furan-2-carboxylate and an alkali metal carbonate. With these components, the reaction was found to proceed efficiently when Cs⁺ salts were used, as shown in FIG. 2a and Extended Data Table 1. When 1 mmol of cesium furan-2-carboxylate and 0.55 mmol Cs₂CO₃ were heated at 260° C. under a CO₂ flow of 40 mL min⁻¹ in a tube furnace, FDCA²⁻ was formed in 76% yield after 12 h, as shown in Entry 2 of Extended Data Table 1. The mass balance was composed of unreacted starting material and decomposition products including acetate. Reactions performed in a Parr reactor showed improved yields and less decomposition. In 1 mmol-scale reactions at 200° C. under 8 bar CO₂, FDCA²⁻ was formed in a 77% yield after 2 h and 89% after 5 h, with only 5% decomposition products, as shown in Entries 5 and 6 of Extended Data Table 1. In 10 mmol-scale reactions under similar conditions, FDCA²⁻ was formed in a 78% yield after 5 h and 81% yield after 10 h, as shown in Entries 8 and 9 of Extended Data Table 1. Further increasing the reaction time did not significantly increase the FDCA²⁻ yield, while increasing the temperature diminished the yield because of increased decomposition. Increasing the CO₂ pressure slowed the reaction by sequestering CO₃ ²⁻ in the form of HCO₃ ⁻. Finally, a 100 mmol-scale reaction was performed under ˜1 bar CO₂ in a rotating round-bottom flask in a 260° C. bath. After 48 h, FDCA²⁻ was formed in a 71% yield, as shown in Entry 12 of Extended Data Table 1. The scaling behavior suggests that the reaction takes place at the molten salt-CO₂ interface. The reaction slows and the yield decreases somewhat as the scale is increased because the surface area-to-volume ratio decreases. Improved yields and rates are anticipated with reactors that disperse the salt more effectively.

TABLE 1 Extended Data Scale Time Start Ent mmol T ° C. p(CO₂) h FDCA²⁻ mat. acetate mal. other 1 1 260 flowing 6 57% 26%  4% 4% 9% 2 1 260 flowing 12 76% 8% 4% 4% 8% 3 1 260 flowing 20 76% 8% 5% 3% 8% 4 1 270 flowing 4 66% 10%  7% 8% 9% 5 1 200 8 bar 2 77% 18%  2% 1% 2% 6 1 200 8 bar 5 89% 6% 3% 2% — 7 1 200 8 bar 7 89% 4% 2% 1% 4% 8 10 195 8 bar 5 78% 11%  4% 1% 6% 9 10 195 8 bar 10 81% 8% 4% 1% 6% 10 10 205 8 bar 2 71% 7% 9% 3% 10%  11 10 215 8 bar 2 69% 2% 14%  5% 10%  12 100 260 1 bar 48 71% 3% 11%  2% 1%

Carboxylation using salts with cations other than Cs⁺ can be achieved by incorporating another carboxylate salt or using cation mixtures. For example, heating K⁺ furan-2-carboxylate with 0.5 equivalents of K₂CO₃ and 1 equivalent of K⁺ isobutyrate at 320° C. under 40 mL min⁻¹ CO₂ resulted in 62% potassium FDCA²⁻, as shown in FIG. 2b . Similar results were obtained with K⁺ acetate as an additive. Thus, C—H carboxylation does not require Cs⁺ per se, but Cs⁺ salts typically have lower melting points.

While the carboxylation results are consistent with the mechanistic scheme in FIG. 1a , there are other possible mechanisms that do not involve a carbanion intermediate. To probe the acid-base properties of furan-2-carboxylate independently, an isotope exchange experiment was performed between furan-2-carboxylate and acetate. (The pKa of acetate is >33). A mixture of 1 mmol cesium furan-2-carboxylate, 1 mmol CD₃CO₂Cs, and 1.1 mmol Cs₂CO₃ was heated under N₂ in the Parr reactor to 200° C. for 1 h. ¹H, ²H, ¹³C NMR and high-resolution mass spectrometry of the crude product mixture showed H/D scrambling between acetate and the 5 position of furan-2-carboxylate and, to a lesser extent, the 3 and 4 positions. The H content remaining in furan-2-carboxylate indicated that the exchange was ˜60% complete (FIG. 2c ). When a 1:1 mixture of cesium furan-2-carboxylate and CD₃CO₂Cs was heated in the absence of Cs₂CO₃ at 200° C., ˜15% H/D exchange was observed, with nearly all of the exchange occurring at the 5 position (FIG. 2c ). Thus, at 200° C. in a molten salt, a carboxylate is able to deprotonate the C5 position of furan-2-carboxylate, and CO₃ ²⁻ is able to deprotonate all positions. The selectivity seen in the carboxylation reaction suggests a greater abundance of the carbanion that leads to FDCA²⁻.

Additional substrates were evaluated to gain further insight into the CO₃ ²⁻ -promoted C—H carboxylation reaction. Heating the Cs⁺ salt of thiophene-2-carboxylate with 0.55 equivalents of Cs₂CO₃ under flowing CO₂ at 325° C. resulted in 71% formation of thiophene-2,5-dicarboxylate after 4 h. This substrate required a significantly higher temperature than furan-2-carboxylate in order to form a semi-molten solution. To see if C—H carboxylation is possible for significantly weaker acids, we evaluated the reactivity of benzoate. Remarkably, heating Cs⁺ benzoate with 0.55 equivalents of Cs₂CO₃ to 320° C. under 8 bar CO₂ resulted in a combined yield of 66% for a mixture of phthalates and tri- and tetracarboxylates (FIG. 3a ). The ability of CO₃ ²⁻ to deprotonate the C—H bonds of a phenyl ring was tested independently by heating a mixture of ¹²C₆D₅CO₂Cs, ¹³C₆H₅CO₂Cs, and 0.55 equivalents of Cs₂CO₃ to 320° C. under N₂ for 30 min. ¹H NMR analysis of the products revealed H/D scrambling at all positions on the benzoate ring, as shown in FIG. 3b . No H/D exchange was observed in the absence of Cs₂CO₃.

The results with benzoate suggest that benzene would undergo C—H carboxylation if exposed to CO₃ ²⁻ in a molten salt. Heating Cs₂CO₃ under benzene and CO₂ at the temperatures up to 380° C. resulted in no reaction. The lack of reactivity can be attributed to the fact that Cs₂CO₃ does not melt. To provide a molten component, reactions were performed in the presence of Cs⁺ isobutyrate. Heating 1.5 mmol Cs₂CO₃ and 1 mmol Cs⁺ isobutyrate to 340-380° C. under 31 bar CO₂ and 42 bar benzene resulted in the formation of benzoate, phthalates, and benzene tricarboxylates, as shown in FIG. 3c . The amount of benzene carboxylation products corresponded to 12% of the Cs₂CO₃ at 350° C., and 19% at 360° C. In addition to benzene carboxylation, isobutyrate carboxylation to dimethyl malonate and decomposition to formate and acetate also occurred under these conditions. Extended Data Table 2 shows the results from the variation of various parameters. The carboxylation of benzene is more challenging than benzoate because there is a larger entropic penalty and the solubility of benzene in the carboxylate salt is likely to be very low. Nevertheless, these results demonstrate that CO₃ ²⁻-promoted hydrocarbon carboxylation is possible.

TABLE 2 Extended Data Time p(C₆H₆) p(CO₂) CO₃ ²⁻ Ent h T ° C. bar bar Conv. A B C D E 1 10 340 13 31 5% 2 14 14 205 17 2 9 340 42 31 9% 42 29 11 238 24 3 2 350 42 31 2% 19 3 1 27 9 4 8 350 42 31 12%  46 41 17 148 28 5 8 360 45 32 19%  39 69 35 306 42 6 1 380 42 33 9% 30 36 13 192 146 A is μmoles of benzonate. B is μmoles of phthalates. C is μmoles of benzene tri + tetracarboxylates. D is μmoles acetate + carboxylation products. E is μmoles formate. D and E correspond to isobutyrate decomposition products.

Scalable CO₃ ²⁻-promoted C—H carboxylation requires facile product isolation and highly efficient recovery of the alkali cation. To illustrate the ease of product isolation in the case of FDCA synthesis, treatment of crude cesium FDCA²⁻ from a C—H carboxylation reaction with 3 N HCl affords immediate precipitation of FDCA, leaving CsCl in the aqueous solution with >99% Cs⁺ recovery, as shown in FIG. 4. To regenerate the CO₃ ²⁻ necessary for C—H carboxylation, bipolar membrane electrodialysis could be used to convert CsCl into HCl and CsOH solutions and the CsOH could be combined with pure CO₂, ambient air, or flue gas to form Cs₂CO₃. HCl is recycled for the protonation step, while CsOH is reacted with 2-furoic acid and CO₂ to generate the starting material for C—H carboxylation. The same procedure could be used with any alkali cation and many other anions, such as sulfate SO₄ ²⁻. The energy requirement for converting aqueous alkali chloride solutions into HCl and alkali hydroxide solutions is ˜0.08 kWh per mol of alkali chloride, which would correspond to ˜1 kWh per kg of FDCA. While additional energy would be required for water removal in each cycle, using highly concentrated solutions would minimize this requirement. The overall process would convert 2-furoic acid into FDCA without using any organic solvents or product distillation steps.

The ability to deprotonate unactivated C—H bonds opens the possibility of using this approach to prepare numerous high-volume and/or high-value targets. In another embodiment, CO₃ ²⁻-promoted carboxylation described here could be used to carboxylate alkyl groups attached to aromatic rings, which have pKa values around 40.

The principle utilized in the embodiments described above could further be extended to CO₂ hydrogenation reactions, where CO₂ is combined with H₂ to make organic molecules. The available methods for CO₂ hydrogenation, which all involve transition metal catalysts, produce relatively low-value Cl compounds such as CH₄ and CO. In another embodiment, CO₃ ²⁻-promoted carboxylation could be used to hydrogenate CO₂ to form compounds with more than one carbon. Since the pKa of H₂ is 35 (Kelly et al. Phys. Chem. Chem. Phys., 3, 2086-2090 (2001)), the embodiments described above show that CO₃ ²⁻ is capable of deprotonating H₂ at elevated temperature. CO₃ ²⁻-promoted carboxylation of H₂ forms formate (HCO₂ ⁻). Formate could further react to form oxalate, enabling CO₃ ²⁻ -promoted hydrogenation of CO₂ to oxalate. The thermochemical transformation of formate to oxalate is known (Andresen J. Org. Chem. 42, 2790 (1977)). In the presence of CO₂, H₂ and carbonate, oxalate could be reduced further to produce other two-carbon compounds such as acetate, ethylene, ethane, ethanol or ethylene glycol. Alternatively, formate could react with CO₂, H₂, and carbonate to produce these same products via a different intermediate.

In the specification and claims a carboxylate salt is a salt comprised of an anion that is an organic compound with a deprotonated carboxylic acid (also referred to as carboxylate) and a cation that is an alkali cation, alkaline earth cation, or other metal cation. An aromatic dicarboxylic acid is defined as a molecule with an aromatic ring that has two carboxylic acid substituents. An example would be phthalic acid (1,2-benzenedicarboxylic acid). An aromatic tricarboxylate salt is defined as a salt with a molecule with an aromatic ring that has 3 carboxylate substituents and associated cations. An example is cesium 1,3,5-benzene tricarboxylate. An Aryl acetate or aryl malonate is a molecule with an aromatic ring that has a 1-carboxyalkyl substituent or 1,1-dicarboxyalkyl substituent. An example would be cesium 3-(carboxylatomethyl)benzoate (an aryl acetate); cesium 2-(3-carboxylatophenyl)malonate (an aryl malonate).

EMBODIMENTS OF IMPLEMENTATION

Some of the above embodiments provide a method for converting a carboxylate substrate into a dicarboxylate that has much greater value or utility than the carboxylate substrate. Some of the above embodiments provide a method of forming organic molecules from a hydrocarbon substrate without a carboxylate, such as benzene or toluene. In these embodiments, CO₃ ²⁻ is used to depronate a carbon hydrogen bond of the substrate to form the conjugate base of the substrate. CO₂ is used to react with the conjugate base to add a carboxylate to the substrate. In some embodiments, a carboxylate salt that other than the substrate is added to the reaction to promote melting. In some embodiments, Cs⁺ is the cation for the carboxylate salt(s) and CO₃ ²⁻. In other embodiments, other cations or mixtures of cations are used including the alkali cations, alkaline earth cations, or other metal cations.

It will be obvious to those with skill in the art that conditions under which CO₃ ²⁻ promotes a carboxylation reaction will also enable the use of bases other than CO₃ ²⁻, such as hydroxide (HO⁻), phosphate (PO₄ ³⁻), or oxide (O²⁻), provided as salts with alkali, alkaline earth, or other cations. In general, the use of CO₃ ²⁻ is preferred because CO₃ ²⁻ salts tend to be less expensive and corrosive than other bases.

Other embodiments may form organic molecules from H₂ and CO₂. In some embodiments, CO₃ ²⁻ is used to depronate the H₂ to form CO₂ is used to react with the H⁻ to form formate. The formate may be further reacted with H₂, CO₃ ²⁻, and/or CO₂ to form compounds with more than one carbon. In some embodiments, a carboxylate salt is added to the reaction to provide a molten salt. In some embodiments, Cs⁺ is the cation for the carboxylate salt(s) and CO₃ ²⁻. In other embodiments, other cations or mixtures of cations are used including the alkali cations, alkaline earth cations, or other metal cations. In some embodiments, CO₃ ²⁻ in the form of a salt with alkali, alkaline earth or other metal cations, is combined with H₂, CO₂, and H₂O and heated to an elevated temperature at an elevated pressure. In some embodiments, a carboxylate salt is added to the CO₃ ²⁻ to provide a molten component. Preferably, the mixture is heated to a temperature between 150° C. and 450° C. More preferably, the mixture is heated to a temperature between 200° C. and 400° C. Most preferably, the mixture is heated to a temperature between 250° C. and 350° C. Preferably, the elevated pressure is between 1 atm and 200 atm. More preferably, the elevated pressure is between 10 atm and 100 atm.

Other embodiments provide a method for synthesizing furan-2, 5-dicarboxylate (FDCA²⁻). A salt comprising furan-2-carboxylate and CO₃ ²⁻ are mixed to form a mixture, which converts the furan-2-carboxylic acid to furan-2-carboxylate. In some embodiments, an additional carboxylate salt is added. CO₂ gas is provided to the mixture. The mixture may be heated to at least partially melt the furan-2-carboxylate. Both of the salts may have cesium cations. The salts may have a mixture of Cs⁺ and K⁺ cations. Other embodiments may only have K⁺ cations. Other embodiments may have mixtures of cations including alkali cations, alkaline earth cations or other metal cations. Preferably, the mixture is heated to a temperature between 100° C. and 450° C. More preferably, the mixture is heated to a temperature between 200° C. and 350° C. Most preferably, the mixture is heated to a temperature between 200° C. and 300° C.

In other embodiments, aromatic tricarboxylate salts are formed by providing a mixture of an aromatic dicarboxylic acid and a CO₃ ²⁻ salt, which converts the aromatic dicarboxylic acid to aromatic dicarboxylate. The mixture is heated under CO₂ gas to a temperature sufficient to at least partially melt the aromatic dicarboxylate. Preferably, the mixture is heated to a temperature between 150° C. and 450° C. More preferably, the mixture is heated to a temperature between 200° C. and 350° C. Most preferably, the mixture is heated to a temperature between 250° C. and 300° C. The aromatic dicarboxylic acid preferably is phthalic acid or is an isomer of phthalic acid, isophthalic acid, or terephthalic acid and a product is a benzene tricarboxylic acid. An additional carboxylate salt may be provided to aid melting at a lower temperature. Preferably, the aromatic dicarboxylic acid is combined with more than one equivalent of a CO₃ ²⁻ salt to form a mixture of the aromatic dicarboxylate salt and a CO₃ ²⁻ salt prior to heating under CO₂.

In other embodiments aryl acetate and aryl malonate salts are formed from a substrate with an aromatic ring substituted with an alkyl group and a carboxylic acid. A CO₃ ²⁻ salt is provided to form a mixture, which converts the carboxylic acid to a carboxylate. The substrate and the CO₃ ²⁻ salt are heated under CO₂ gas to a temperature sufficient to at least partially melt the carboxylate. Preferably, the substrate is combined with more than one-half equivalent of a CO₃ ²⁻ salt to form a mixture of a carboxylate salt and a CO₃ ²⁻ salt prior to heating under CO₂. Preferably, the mixture is heated to a temperature between 150° C. and 450° C. More preferably, the mixture is heated to a temperature between 200° C. and 350° C. Most preferably, the mixture is heated to a temperature between 250° C. and 300° C. In an embodiment, the substrate is an isomer of methylbenzoic acid. In an embodiment, an additional carboxylate salt is provided to aid melting at a lower temperature.

In other embodiments carboxylate salts are formed. An organic molecule without a carboxylate is provided. A CO₃ ²⁻ salt is provided. A molten salt is provided. CO₂ gas is provided to the mixture of the organic molecule, the CO₃ ²⁻ salt, and molten salt. The mixture is heated to maintain the molten salt. Preferably, the molten salt has Cs⁺ cations. More preferably, the molten salt has carboxylate anions.

In the specification and claims a molten salt is a salt with a low enough melting point to be at least partially melted at a reaction temperature. Preferably, the molten salt has a melting point between 150° C. and 450° C. More preferably, the molten salt has a melting point between 200° C. and 350° C. Most preferably, the molten salt has a melting point between 250° C. and 300° C. When providing the molten salt, in some embodiments the molten salt is partially melted. In other embodiments, the molten salt may be provided in solid form and the subsequently be at least partially melted. An example of a molten salt is Cs⁺ isobutyrate.

In the specification and claims when comparing amounts of elements or molecules, unless otherwise specified, the number of moles of the element or molecule is being compared.

While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, modifications and various substitute equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, modifications, and various substitute equivalents as fall within the true spirit and scope of the present invention. 

What is claimed is:
 1. A method for synthesizing a dicarboxylate compound containing an aromatic heterocycle, comprising: providing a carboxylic acid containing an aromatic heterocycle; providing a CO₃ ²⁻ salt with Cs⁺ cations to form a mixture, which converts the carboxylic acid containing an aromatic heterocycle to a carboxylate containing an aromatic heterocycle; providing a CO₂ gas to the mixture; and heating the mixture to a temperature to at least partially melt the carboxylate containing an aromatic heterocycle.
 2. The method as recited in claim 1, where the dicarboxylate compound is thiophene-2, 5-dicarboxylate, and wherein the carboxylic acid containing an aromatic heterocycle is thiophene-2-carboxylic acid, and wherein the carboxylate containing an aromatic heterocycle is thiophene-2-carboxylate.
 3. The method as recited in claim 1, where the carboxylic acid containing an aromatic heterocycle and more than one-half equivalent of CO₃ ²⁻ salt are combined to form the mixture of carboxylic acid containing an aromatic heterocycle salt and CO₃ ²⁻ salt prior to heating under CO₂.
 4. The method as recited in claim 1, wherein the mixture has K⁺ cations.
 5. The method as recited in claim 1, wherein the mixture only has K⁺ cations and further comprising providing an additional carboxylate salt with K⁺ cations, to aid melting at a lower temperature.
 6. The method as recited in claim 1, where the temperature is between 100° C. and 450° C.
 7. The method as recited in claim 1, where the temperature is between 200° C. and 350° C.
 8. The method as recited in claim 1, where the temperature is between 200° C. and 300° C.
 9. The method, as recited in claim 1, further comprising isolating and recovering the Cs⁺ anions.
 10. The method, as recited in claim 9, wherein the isolating and recovering the Cs⁺ anions, comprises: generating a CsCl aqueous solution; using a bipolar membrane electrodialysis to covert CsCl into HCl and CsOH solutions; and combining the CsOH solution with CO₂ gas to form Cs₂CO₃. 